In an internal combustion engine it is generally necessary to control a mixing ratio of air and fuel in a mixed gas burned in the internal combustion engine to control the air-fuel ratio. Such control provides efficient catalytic purification of hazardous components present in the exhaust gas. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture. In order to perform such control of air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust gas passage of the internal combustion engine and feedback control is implemented such as to match the air-fuel ratio detected by the sensor with a predetermined target air-fuel ratio.
In a multi-cylinder internal combustion engine, the air-fuel ratio control is usually performed by using the same control amount for all of the cylinders. Therefore, the actual air-fuel ratio can vary between the cylinders even when the air-fuel ratio control is implemented. Furthermore, while the fueling provided to each cylinder can be adjusted and controlled to be the same across cylinders, other factors can contribute to cause inconsistencies between the cylinders. Such other factors include things such as charge (air+any recirculated exhaust) composition, start of injection timing, and cylinder/piston geometry.
Such inter-cylinder imbalances have the potential to cause torque pulsations, reduce efficiency, and increase emissions and component wear.
What is therefore needed is a system for detecting inter-cylinder imbalances and then taking affirmative steps to reduce the imbalances.